Golden orb web spiders (Nephila antipodiana) are a common species found all over the world, particularly in tropical areas including in Southeast Asia and Australia.

They weave giant webs over 1 metre across, catching all manner of insects from flies to cicadas for their supper.

These spiders - and the insects they catch in their web - are potential prey for ants, says researcher Professor Mark Elgar, an evolutionary biologist at the University of Melbourne, so it's surprising that ants are never found foraging on the webs of these spiders.

"The paradox is resolved by this chemical that's found on the silk that keep ants at bay," he says.

"It's a bit like body odour on a cramped bus on a hot day. It's not going to kill you, but it's certainly going to get you off the bus pretty quickly."

Elgar says the repellent chemical - a pyrrolidine alkaloid - is not a byproduct of silk production but it is produced by insects to counter predation.

"We don't know exactly how the spiders produce such a compound," says co-researcher, Professor Daiqin Li, and expert in animal behaviour from the National University of Singapore.

He says it's possible that the spiders obtain the compound by eating insects that contain it.

Experiments

Li says the team extracted the chemical from the spider silk using a solvent and then used GCMS (gas chromatography-mass spectrometry) to identify it as a pyrrolidine alkaloid, which is a already known about by chemists and used by them.

They then tested the chemical's role in keeping the ants at bay.

The researchers used bait to attract ants towards the spider silk, in three different experiments.

In one case, the natural silk was used, complete with the pyrrolidine alkaloid. In the second case, the silk had all the chemical removed. In the third case, purified pyrrolidine alkaloid purchased from chemical supplies was placed on the cleaned silk.

Li says the ants were able to cross the cleaned silk but were unable to cross in the two cases where pyrrolidine alkaloid was present.

He says collaborators are trying to reformulate the liquid chemical into a powder form that might be more useful in pest control.

While the chemical occurs on silk threads produced by adult and large juvenile spiders, it is not on the silk produced by small juvenile spiders, which are not under threat of invasion from ants, say the researchers.

"Their silk is so thin that ants can't walk onto it," says Elgar.

He suspects the chemical would not be a deterrent for large ants which are too heavy to walk on the silk.